A fiber array spectral translator (“FAST”) system when used in conjunction with a photon detector allows massively parallel acquisition of full-spectral images. A FAST system can provide rapid real-time analysis for quick detection, classification, identification, and visualization of the sample. The FAST technology can acquire a few to thousands of full spectral range, spatially resolved spectra simultaneously. A typical FAST array contains multiple optical fibers that may be arranged in a two-dimensional array on one end and a one dimensional (i.e., linear) array on the other end. The linear array is useful for interfacing with a photon detector, such as a charge-coupled device (“CCD”). The two-dimensional array end of the FAST is typically positioned to receive photons from a sample. The photons from the sample may be, for example, emitted by the sample, reflected off of the sample, refracted by the sample, fluoresce from the sample, or scattered by the sample. The scattered photons may be Raman photons.
In a FAST spectrographic system, photons incident to the two-dimensional end of the FAST may be focused so that a spectroscopic image of the sample is conveyed onto the two-dimensional array of optical fibers. The two-dimensional array of optical fibers may be drawn into a one-dimensional distal array with, for example, serpentine ordering. The one-dimensional fiber stack may be operatively coupled to an imaging spectrograph of a photon detector, such as a charge-coupled device so as to apply the photons received at the two-dimensional end of the FAST to the detector rows of the photon detector.
One advantage of this type of apparatus over other spectroscopic apparatus is speed of analysis. A complete spectroscopic imaging data set can be acquired in the amount of time it takes to generate a single spectrum from a given material. Additionally, the FAST can be implemented with multiple detectors. The FAST system allows for massively parallel acquisition of full-spectral images. A FAST fiber bundle may feed optical information from its two-dimensional non-linear imaging end (which can be in any non-linear configuration, e.g., circular, square, rectangular, etc.) to its one-dimensional linear distal end input into the photon detector. Given the advantageous ability of a FAST system to acquire hundreds to thousands of full spectral range, spatially-resolved spectra, such as Raman spectra, substantially simultaneously, a FAST system may be used in a variety of situations to help resolve difficult spectrographic problems such as the presence of polymorphs of a compound, sometimes referred to as spectral unmixing.
Chemical images may generally be acquired using one of two classes of approaches: (1) scanning, and (2) widefield chemical imaging. In scanning methods, a radiation source is focused onto the surface of a sample and a spectrum from each spatial position is collected using a dispersive spectrograph or interferometer. Long data collection times are common with scanning methods since the duration of the experiment is proportional to the number of image pixels. Because of such long data collection times, scanned images are captured at low image definition, which relates directly to the limited utility of the technique as an imaging tool, for the routine assessment of material morphology. Furthermore, the spatial resolution of the image is limited by the size of the source illumination on the sample and the rastering mechanism, which requires the use of moving mechanical parts that are challenging to operate reproducibly. In addition, for light-absorbing materials, scanning methods present an enormous challenge. These materials have low damage thresholds, requiring the use of low laser power densities to minimize local thermal expansion and sample degradation.
Despite the limitations, scanning methods are relatively mature techniques and have been applied in a number of applications. An advantage of scanning-based chemical imaging is the ability to capture the entire spectrum in an efficient manner. This advantage is best realized in the research evaluation of new material systems where the underlying spectroscopy is not well understood, and therefore, benefits may be available from the analysis of the entire spectrum. In widefield chemical imaging, the entire sample field of view is illuminated and analyzed simultaneously. Numerous widefield chemical imaging approaches have been demonstrated, with the majority of methods involving the recording of an image at discrete spectral intervals though an imaging spectrometer (i.e., LCTF (Liquid Crystal Tunable Filter), AOTF (Acousto-Optic Tunable Filter), etc.).
Because both (X-Y) spatial dimensions are collected simultaneously in widefield Chemical Imaging using imaging spectrometers, the experiment duration is proportional to the number of spectral channels and not to the number of image pixels. The widefield advantages are best realized when high fidelity images at a limited number of wavelengths provide sufficient chemical and spatial information. In most materials characterization applications, only a limited number of spectral bands (typically <100) are required to analyze the analytes of interest. By reducing the number of spectral channels, the duration of the widefield experiment decreases without losing spatial resolution. In addition, time-dependent changes in the sample are only observed in the spectral dimension, which simplifies the analysis of chemical images in widefield imaging.
Conversely, attempts to reduce the duration of scanning experiments (in the scanning approach discussed above) compromise either the spatial resolution or the field of view. Reducing the number of spectral channels in scanning mode has little effect on the experiment duration since, the entire chemical spectrum is captured simultaneously (in the scanning approach discussed above). Scanning experiments record time dependent sample changes as spatial variations. Pixels collected at different times often have induced spectral differences that complicate analysis.
A limitation that widefield illumination methods suffer from is secondary scattering of illumination that fundamentally reduces the inherent confocality associated with the measurement(s). Secondary scattering occurs when illumination of a first location results in an emission or scattering of radiation that migrates to a second sample location and is detected as if it had originated in the second spatial location. On the other hand, the scanning approach discussed above is generally less susceptible to secondary illumination effects since the illumination is first restricted to a first sample location and the collected light is then restricted to the same sample location through use of pinhole apertures. Line scanning approaches are slightly more susceptible to secondary scattering effects along the sample axis that is aligned in parallel with the entrance slit of the spectrograph compared to point mapping/scanning approaches. FAST enables full spectral acquisition for hundreds to thousands of spatially resolved spectra in a single image frame—dramatically increasing data acquisition rates compared to current tunable filter based technologies. Software is used to extract the spatial/Spectral information to reconstruct hyperspectral (chemical imaging) data cubes of the original object. Furthermore, FAST is a rugged technology that operates over an extensive spectral range from ultraviolet (UV) to infrared (IR).
As with alternative widefield chemical imaging methods, FAST based systems are susceptible to secondary scattering of radiation. The present disclosure describes systems and methods for overcoming the limitations, of the prior art including novel sample illumination and light collection systems and methods to enhance the confocality of FAST based spectroscopy systems. The present disclosure also describes systems and methods that use telescope optics to allow for detection at greater distances than those described in the prior art.